Amidoxime modified Aspergillus niger (AMAN) was prepared by the oximation reaction. The effects of the initial pH, contact time, initial U(VI) concentration and biosorbent dose on the adsorption of U(VI) ions from radioactive wastewater in U(VI) concentrations of less than 1 mg L⁻¹ by AMAN and the raw Aspergillus niger (RAN) were investigated. The maximum adsorption efficiency by AMAN for the 0.5 mg L⁻¹ U(VI) solution amounted to 98.85% under the optimum adsorption conditions, while the maximum adsorption efficiency by RAN was only 77.83%. The adsorption equilibrium data were found to be best fitted to Langmuir isotherm model, and the maximum biosorption capacity of AMAN for U(VI) was estimated to be 621 mg g⁻¹ at 298 K. The biosorption kinetics followed the pseudo-second order model and intraparticle diffusion equation. The Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) showed that the adsorption process of U(VI) was spontaneous, feasible and endothermic. The SEM-EDS study indicated that much more U(VI) ions were adsorbed by AMAN than by RAN. FT-IR study showed that the –NH₂ and N–OH groups of amidoxime were the dominant ones for binding UO₂ ²⁺ ions. Moreover, AMAN was found to have excellent selective adsorption capability of U(VI) due to amidoxime groups. The UO₂ ²⁺ ions adsorbed by AMAN could be desorbed using 0.1 M HCl, and the desorption efficiency reaching 87.28% at the 8th cycle of adsorption and desorption.